Re: [dm-devel] [PATCH] blk-lib: fix error reporting

Mikulas Patocka
2014-07-08T13:05:36Z

On Tue, 8 Jul 2014, Christoph Hellwig wrote:

> > + if (unlikely(err))
> > + ACCESS_ONCE(bb->error) = err;
>
> I can't see a reason for the ACCESS_ONCE here.

Multiple bios can be completed concurrently, so they write bb->error at
the same time. The compiler may do store tearing (see "store tearing" in
Documentation/memory-barriers.txt) - it may split one 4-byte write into
several smaller writes - and it could result in setting bb->error to
invalid value. We need ACCESS_ONCE to make sure that store tearing doesn't
happen.

Mikulas

> Also the likely/unlikely annotations here smell like premature
> optimization.
>
> Otherwise looks good to me.